Two officers, Guglielmo and Ferrando express certainty about the fidelity of Dorabella and Fiordiligi, two sisters whom they are about to marry. Don Alfonso, their friend and hierarchical superior provokes them by saying that a faithful woman is only a figment of imagination and he holds a wager with the two claiming that he could prove his theory.
Fiordiligi and Dorabella find that Guglielmo and Ferrando have been called off to war. As the two young men leave, Don Alfonso asks Despina for help in order to introduce the two sisters to two of his friends. Believing that they should be free to love whoever they want, whenever they want, Despina agrees to convince them to have fun.
The two friends of Don Alfonso are Guglielmo and Ferrando, disguised, in the attempt to seduce each other’s fiancées. Willing to prove their fidelity Fiordiligi and Dorabella refuse to give in and vehemently reject their advances. Despina takes over the seduction plan and asks the two men to continue their farce. Guglielmo and Ferrando then simulate a suicide attempt and Despina, desguised as a doctor, offers the who sisters her help.
While Despina tries to teach the two sisters how to deal with men, Dorabella wonders if she should not have some fun of the two foreigners. Ferrando and Guglielmo continue their attempted seduction and as a result Dorabella and Guglielmo already become a couple. Fiordiligi tries to oust Ferrando, even though she admits she fell in love with him. Seized with remorse, she decides to go to the army and find her betrothed. When Ferrando finds out that his fiancée, Dorabella, has been seduced by Guglielmo, he becomes even more determined to win over Fiordiligi’s heart. She finally succumbs, while Guglielmo is witness to all these events.
Despina makes the arrangements for a double wedding: Fiordiligi will marry Ferrando and Dorabella will marry Guglielmo. After signing the contracts presented by Despina (disguised as a notary), Don Alfonso announces the return of Guglielmo and Ferrando, finally revealing the two sisters that they had been duped.